1. Field of the Invention
This invention relates generally to amplifier front-end circuits, and, more particularly, to front-end circuits which enable the amplifier to accommodate a common-mode input voltage of zero.
2. Description of the Related Art
An amplifier ‘front-end’ circuit operates to receive and buffer an input signal, and provide a representation of the received signal to a following ‘back-end’ stage. Two front-end circuits may be used to accommodate a differential input signal. The front-end and back-end circuits together provide an instrumentation amplifier.
One type of amplifier is a current-mode instrumentation amplifier. In a typical current-mode instrumentation amplifier, first and second front-end circuits provide respective output currents, and a back-end output stage provides an output which varies with the difference between the output currents. A conventional front-end for this type of amplifier is shown in FIG. 1. A common mode input voltage Vcm is applied to one input of a differential transistor pair 10 biased with a tail current source 12. The pair transistors conduct respective currents, which drive a folded cascode stage. 14 that produces a single-ended output 16. This output is coupled to a second gain stage which includes an output transistor 18 biased with a current source 20, typically a field-effect transistor (FET). Output transistor 18 provides an output signal current 22 to an output stage (not shown).
When arranged as shown in FIG. 1, the voltages at the inputs to the differential pair are forced to be equal, and the voltage at the junction 24 of output transistor 18 and current source 20 tracks that at the pair inputs. Thus, the voltage at junction 24 will be approximately equal to zero when Vcm=0. Ideally, the FET of current source 20 operates in its saturation region. However, a voltage of zero at junction 24 forces the FET to turn off, thereby eliminating the bias current provided to output transistor 18. Therefore, a front-end having a configuration of this sort cannot function accurately when Vcm=0.